EP0197762A2 - MOS capacitor and method of manufacturing the same - Google Patents
MOS capacitor and method of manufacturing the same Download PDFInfo
- Publication number
- EP0197762A2 EP0197762A2 EP86302450A EP86302450A EP0197762A2 EP 0197762 A2 EP0197762 A2 EP 0197762A2 EP 86302450 A EP86302450 A EP 86302450A EP 86302450 A EP86302450 A EP 86302450A EP 0197762 A2 EP0197762 A2 EP 0197762A2
- Authority
- EP
- European Patent Office
- Prior art keywords
- polycrystalline silicon
- silicon film
- insulating film
- semiconductor substrate
- film
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
- 239000003990 capacitor Substances 0.000 title claims abstract description 54
- 238000004519 manufacturing process Methods 0.000 title claims description 13
- 229910021420 polycrystalline silicon Inorganic materials 0.000 claims abstract description 43
- 239000000758 substrate Substances 0.000 claims abstract description 42
- 238000002955 isolation Methods 0.000 claims abstract description 32
- 239000004065 semiconductor Substances 0.000 claims abstract description 26
- 239000012212 insulator Substances 0.000 claims abstract description 7
- 238000009792 diffusion process Methods 0.000 claims description 6
- 239000000463 material Substances 0.000 claims description 6
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 abstract description 17
- 229910052710 silicon Inorganic materials 0.000 abstract description 17
- 239000010703 silicon Substances 0.000 abstract description 17
- 230000007547 defect Effects 0.000 abstract description 6
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 22
- 238000000034 method Methods 0.000 description 17
- 235000012239 silicon dioxide Nutrition 0.000 description 11
- 239000000377 silicon dioxide Substances 0.000 description 11
- 239000013078 crystal Substances 0.000 description 3
- BHEPBYXIRTUNPN-UHFFFAOYSA-N hydridophosphorus(.) (triplet) Chemical compound [PH] BHEPBYXIRTUNPN-UHFFFAOYSA-N 0.000 description 3
- 230000003647 oxidation Effects 0.000 description 3
- 238000007254 oxidation reaction Methods 0.000 description 3
- 238000001020 plasma etching Methods 0.000 description 3
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 2
- 229910052782 aluminium Inorganic materials 0.000 description 2
- 230000010354 integration Effects 0.000 description 2
- 238000012986 modification Methods 0.000 description 2
- 230000004048 modification Effects 0.000 description 2
- 238000001259 photo etching Methods 0.000 description 2
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 description 1
- 241000293849 Cordylanthus Species 0.000 description 1
- MYMOFIZGZYHOMD-UHFFFAOYSA-N Dioxygen Chemical compound O=O MYMOFIZGZYHOMD-UHFFFAOYSA-N 0.000 description 1
- 229910052796 boron Inorganic materials 0.000 description 1
- 239000011248 coating agent Substances 0.000 description 1
- 238000000576 coating method Methods 0.000 description 1
- 239000000470 constituent Substances 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 230000007423 decrease Effects 0.000 description 1
- 229910001882 dioxygen Inorganic materials 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000005530 etching Methods 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 230000008520 organization Effects 0.000 description 1
- 230000001590 oxidative effect Effects 0.000 description 1
- 230000001681 protective effect Effects 0.000 description 1
- WFKWXMTUELFFGS-UHFFFAOYSA-N tungsten Chemical compound [W] WFKWXMTUELFFGS-UHFFFAOYSA-N 0.000 description 1
- 229910052721 tungsten Inorganic materials 0.000 description 1
- 239000010937 tungsten Substances 0.000 description 1
Images
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
- H01L21/70—Manufacture or treatment of devices consisting of a plurality of solid state components formed in or on a common substrate or of parts thereof; Manufacture of integrated circuit devices or of parts thereof
- H01L21/71—Manufacture of specific parts of devices defined in group H01L21/70
- H01L21/76—Making of isolation regions between components
- H01L21/762—Dielectric regions, e.g. EPIC dielectric isolation, LOCOS; Trench refilling techniques, SOI technology, use of channel stoppers
- H01L21/76224—Dielectric regions, e.g. EPIC dielectric isolation, LOCOS; Trench refilling techniques, SOI technology, use of channel stoppers using trench refilling with dielectric materials
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
- H01L21/70—Manufacture or treatment of devices consisting of a plurality of solid state components formed in or on a common substrate or of parts thereof; Manufacture of integrated circuit devices or of parts thereof
- H01L21/71—Manufacture of specific parts of devices defined in group H01L21/70
- H01L21/76—Making of isolation regions between components
- H01L21/763—Polycrystalline semiconductor regions
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
- H01L29/86—Types of semiconductor device ; Multistep manufacturing processes therefor controllable only by variation of the electric current supplied, or only the electric potential applied, to one or more of the electrodes carrying the current to be rectified, amplified, oscillated or switched
- H01L29/92—Capacitors with potential-jump barrier or surface barrier
- H01L29/94—Metal-insulator-semiconductors, e.g. MOS
- H01L29/945—Trench capacitors
-
- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10B—ELECTRONIC MEMORY DEVICES
- H10B12/00—Dynamic random access memory [DRAM] devices
- H10B12/01—Manufacture or treatment
- H10B12/02—Manufacture or treatment for one transistor one-capacitor [1T-1C] memory cells
- H10B12/03—Making the capacitor or connections thereto
- H10B12/038—Making the capacitor or connections thereto the capacitor being in a trench in the substrate
Abstract
Description
- Recently, MOS dynamic RAMs (hereinafter referred to as DRAMs) having a large storage capacity on the order of mega-bit have been realized. In keeping with such a trend, it has been designed to achieve higher density by reducing the size of a memory cell.
- The present invention relates to a MOS capacitor and a manufacturing method thereof, and in particular, to a MOS capacitor suitable for increasing the density of a DRAM and a manufacturing method of such a MOS capacitor.
- With the progress in study for increasing the degree of integration of the DRAM, a DRAM of a large capacity as large as several mega-bits has come to be reported. However, since the size of a chip generally increases as the storage capacity is increased, in order to achieve practical use, it is necessary to increase the density by further reducing the size of the memory cell.
- An area of the memory cell per 1 bit is in a range from 50 to 70 µm2 for a DRAM of 256 kb, and the area must be reduced to a range from 20 to 30 pm 2 or less for 1 Mb. However, taking a soft error and noise margin into consideration, it is necessary that a capacitor capacity constituting the memory cell is made substantially comparable to the DRAM of 256 kb.
- In order to maintain the capacity of the capacitor of the memory cell comparable to that of the DRAM of 256 kb while reducing the memory cell area, there is a method in which the effective film thickness of an insulating film (hereinafter referred to as a capacitor insulating film) which is a constituent element of the capacitor is reduced, or another method in which the effective area of the capacitor insulating film is increased.
- However, in the former method, since it is difficult to make a silicon dioxide film thiner than about 10 - 15 pm in view of pinholes and the like, there exists a limitation.
- On the other hand, a trench-shaped capacitor is known in which, in order to eliminate the drawbacks mentioned above, a trench of several pm in depth is formed in the semiconductor substrate, and by forming a capacitor on the inner wall of the trench, the effective area of the capacitor is increased (e.g., Kiyoo Itoh et al, "256 k/1 Mb DRAMS-II", 1984, ISSCC, pages 282 - 288).
- The trench-shaped capacitor of this structure is a kind of the MOS capacitor structure in which an element isolation region is formed in the silicon substrate by a selective oxidation method, trenches are formed adjacent to the element isolation region and sandwiching this region, insulating films are respectively formed on inner walls of these trenches, and the trenches are filled up by forming polycrystalline silicon films on the insulating films so that the insulating films constitute dielectric layers and the silicon substrate and the polycrystalline . silicon films are used as electrodes.
- In the trench-shaped capacitor mentioned above, it is designed to reduce the area of the memory cell by forming the trenches in the silicon substrate and forming the capacitor on the inner walls. However, when the trench-shaped capacitors are formed adjacent to both sides of the element isolation region formed by the selective oxidation method, punch through will be caused between the adjacent capacitors, and reducing the isolation width will become difficult. Moreover, since the selective oxidation method is employed in the element isolation, there arises a problem in that the generation of a bird's beak can not be avoided and higher integration can not be achieved.
- Furthermore, a crystal defect or the like occured in the silicon substrate during the trench forming process and the heat treatment process will cause a leakage in the substrate and a defect in the capacitor insulating film. These problems pose a large obstacle in manufacturing a large capacity memory.
- A first object of the present invention is to provide a MOS capacitor and a manufacturing method in which an element isolation region of high isolation dielectric strength is formed.
- A second object of the present invention is to form an element isolation region which has a reduced isolation width and which is suitable for being made in a microstructure.
- A third object of the present invention is to form an insulating film for a MOS capacitor which is not influenced by a crystal defect occured in the substrate.
- A fourth object of the present invention is to form a MOS capacitor having a large capacity while being formed in a microstructure.
- In the present invention, a capacitor is manufactured through a step of forming a first trench in the main surface of a semiconductor substrate of one conductivity type, after covering an inner wall of the trench with a first insulating film, filling the inside of the first trench with an insulator or a semiconductor material, and forming an element isolation region having the substrate surface flattened; a step of forming at least two trenches of second and third trenches adjacent to both sides of the element isolation region and opposing to each other sandwiching the element isolation region; a step of forming a first polycrystaline silicon film of an opposite conductivity type to that of the semiconductor substrate on the whole of the inner walls of the second and third trenches; a step of forming a second insulating film on the first polycrystalline silicon film; and a step of forming a second conductive polycrystalline silicon film on the second insulating film to fill the second and third trenches, wherein the second insulating film constitutes a dielectric layer and the first and second polycrystalline silicon films serve as electrodes.
- According to the structure realized by the method mentioned above, a MOS capacitor is obtained in which a first set of a first polycrystalline silicon film constituting one electrode, a first insulating film constituting a dielectric layer, and a second polycrystalline silicon film constituting the other electrode are formed side by side, and are embedded in a first portion of the semiconductor substrate in substantially perpendicular directions to the main surface of the semiconductor substrate, and at least a second set of a similar composition to that of the first set is embedded in a second portion of the semiconductor substrate, and an isolation region including an insulator or a semiconductor material, and a first insulating film surrounding the insulator or the semiconductor material is embedded between the first and second portions.
- In the MOS capacitor and the manufacturing method thereof in the present invention, the element isolation region can be formed to provide high isolation dielectric strength and in a trench shape having a reduced width. Furthermore, since 'a polycrystalline silicon film is formed on the inner wall of the trench-shaped capacitor, and an insulating film of the capacitor is formed on the polycrystalline silicon film, the capacitor insulating film can be formed in a region which is not influenced by a crystal defect occured in the substrate.
- While the novel features of the invention are set forth with particularity in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.
-
- Figs. 1 to 10 are sectional views illustrating in the order of processes in a manufacturing method of a DRAM applied with a manufacturing method of a MOS capacitor in accordance with the present invention.
- An embodiment of a manufacturing method of a DRAM applied in order to obtain a structure of a MOS capacitor in accordance with the present invention will be described with reference to a flow chart of a process.
- First, as shown in Fig. 1, a
silicon dioxide film 17 is formed on a surface of a P-type silicon substrate 16, and anopening 18 having a width of 1 pm is formed in a portion intended to be an element isolation region by a photoetching technique. - Next, as shown in Fig. 2, a
trench 19 having a depth of 5 µm and a width of 1 µm is formed in thesilicon substrate 16 by reactive ion etching using CC14 and 02 and by using thesilicon dioxide film 17 as a mask. Following this, achannel stopper region 20 is formed on the bottom of thetrench 19 by ion implanation of boron. - Next, as shown in Fig. 3, after oxidizing thermally the inner wall of the
trench 19 and the surface of thesilicon substrate 16, asilicon dioxide 21 is deposited by a CVD technique to fill thetrench 19 with thesilicon dioxide 21, and anelement isolation region 22 is formed. - In this respect, as a technique for filling the
trench 19, another technique may be used in which after the silicon dioxide is formed on the inner wall of thetrench 19, a polycrystalline silicon layer is formed by the CVD technique thereby to fill thetrench 19 with this polycrystalline silicon layer. Following this, as shown in Fig. 4, thesilicon dioxide films substrate 16 are removed entirely by etching. - Next, as shown in Fig. 5, a new
silicon dioxide film 23 is formed on the surface of thesilicon substrate 16, and further, by the photoetching technique, openings for forming trench-shaped capacitors are formed at areas located at both sides of theelement isolation region 22 and adjoining thisregion 22. Following this, the reactive ion etching is applied to silicon substrate portions exposed at the inside of these openings, and treches 24 and 24 each having a depth of 4 µm and a width of 1 µm are formed in thesilicon substrate 16. Thesetrenches element isolation region 22 and are of a construction adjoining theelement isolation region 22. - Next, as shown in Fig. 6, an N-conductivity type
polycrystalline silicon film 25 containing phosphorous (P) is grown to a film thickness of about 200 nm on the inner walls of thetrenches type diffusion layers polycrystalline silicon film 25 to thesilicon substrate 16, and thesilicon substrate 16 and thepolycrystalline silicon film 25 are electrically connected. - Next, after coating a resist (not shown) all over the
polycrystalline silicon film 25 to fill thetrenches trenches polycrystalline silicon film 25 and thesilicon dioxide film 23 on the surface are sequentially removed. Thereafter, the resist in the inside of thetrenches - Next, as shown in Fig. 8, a
silicon dioxide film 27 is formed so that an effective oxide film thickness becomes 15 nm as a capacitor insulating film. On thissilicon dioxide film 27, apolycrystalline silicon film 28 containing phosphorous and intended to constitute the other electrode of a capacitor is formed to a thickness of 500 nm, and the surface is flattened by filling the trenches each having a width of 1 um. - Next, as shown in Fig. 9, the
polycrystalline silicon film 28 on the surface is selectively removed, and an electrode pattern for the capacitor is formed. Following this, agate oxide film 29 of an access MOS transistor is formed, and further, alayer insulating film 30 for insulating the capacitor electrode from a word line is formed. Then, after agate electrode 31 made of tungsten or aluminum and the word line connected to thegate electrode 31 are formed, N-type diffusion regions - Next, as shown in Fig. 10, a layer
insulating film 35 for insulating theword line 32 from a bit line is formed, and hereafter, an opening for forming an electrode is formed in the N-type diffusion region 33, and an aluminum wiring is selectively formed so that one end of thebit line 36 is connected to this opening. - Lastly, a memory cell of a DRAM is completed by forming a protective coat (not shown).
- In this respect, the N-
type diffusion layer 26 formed on the substrate side of the inner wall of eachtrench 24 shown in the embodiment may be omitted, and in this case, although the capacitor capacity decreases by about 10%, the effect of reduction of the memory cell area is not changed. - The memory cell of the DRAM formed by the MOS capacitor and the manufacturing method thereof in accordance with the present invention provides a capacity of 60 fF although the element isolation width and the trench width of the capacitor are reduced to 1 µm.
- Moreover, even when the element isolation trench width is made 1 um, a substrate resistivity is about 4 - 5 n cm, and the dielectric strength between elements of 20 V can also be obtained.
- Further, when this manufacturing method is used for a DRAM, since an insulating film is formed on a polycrystalline silicon film after the polycrystalline silicon film is formed on the inner wall of a trench for forming a capacitor, the capacitor insulating film for the DRAM is not influenced by a defect occured in the silicon substrate due to work strain or the like. Thus, a DRAM having high dielectric strength, and still having an improved characteristic of a time (pause time) for the charges stored in the capacitor naturally disappear due to leakage can be obtained.
- While specific embodiments of the invention have been illustrated and described herein, it is realized that modifications and changes will occur to those skilled in the art. It is therefore to be understood that the appended claims are intended to cover all modifications and changes as fall within the true spirit and scope of the invention.
Claims (8)
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP60070278A JP2604705B2 (en) | 1985-04-03 | 1985-04-03 | Method of manufacturing MOS capacitor |
JP70278/85 | 1985-04-03 |
Publications (3)
Publication Number | Publication Date |
---|---|
EP0197762A2 true EP0197762A2 (en) | 1986-10-15 |
EP0197762A3 EP0197762A3 (en) | 1987-08-19 |
EP0197762B1 EP0197762B1 (en) | 1991-06-12 |
Family
ID=13426872
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP86302450A Expired EP0197762B1 (en) | 1985-04-03 | 1986-04-02 | Mos capacitor and method of manufacturing the same |
Country Status (4)
Country | Link |
---|---|
US (1) | US4797719A (en) |
EP (1) | EP0197762B1 (en) |
JP (1) | JP2604705B2 (en) |
DE (1) | DE3679698D1 (en) |
Cited By (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0317152A2 (en) * | 1987-11-13 | 1989-05-24 | Fujitsu Limited | Trench capacitor and method for producing the same |
Families Citing this family (12)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JP2590867B2 (en) * | 1987-03-27 | 1997-03-12 | ソニー株式会社 | Manufacturing method of memory device |
JPH01287956A (en) * | 1987-07-10 | 1989-11-20 | Toshiba Corp | Semiconductor memory and manufacture thereof |
US4896293A (en) * | 1988-06-09 | 1990-01-23 | Texas Instruments Incorporated | Dynamic ram cell with isolated trench capacitors |
US4958318A (en) * | 1988-07-08 | 1990-09-18 | Eliyahou Harari | Sidewall capacitor DRAM cell |
US5143861A (en) * | 1989-03-06 | 1992-09-01 | Sgs-Thomson Microelectronics, Inc. | Method making a dynamic random access memory cell with a tungsten plug |
KR920004028B1 (en) * | 1989-11-20 | 1992-05-22 | 삼성전자 주식회사 | Semiconductor devices and its manufacturing method |
US5256588A (en) * | 1992-03-23 | 1993-10-26 | Motorola, Inc. | Method for forming a transistor and a capacitor for use in a vertically stacked dynamic random access memory cell |
US5429978A (en) * | 1994-06-22 | 1995-07-04 | Industrial Technology Research Institute | Method of forming a high density self-aligned stack in trench |
US6222218B1 (en) | 1998-09-14 | 2001-04-24 | International Business Machines Corporation | DRAM trench |
EP0996149A1 (en) * | 1998-10-23 | 2000-04-26 | STMicroelectronics S.r.l. | Manufacturing method for an oxide layer having high thickness |
JP3580719B2 (en) * | 1999-03-03 | 2004-10-27 | 株式会社東芝 | Semiconductor storage device and method of manufacturing the same |
KR20070105710A (en) * | 2006-04-27 | 2007-10-31 | 윤욱현 | Mos capacitor and method of manufacturing the same |
Citations (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0118878A2 (en) * | 1983-03-07 | 1984-09-19 | Hitachi, Ltd. | Semiconductor memory device |
GB2138207A (en) * | 1983-04-15 | 1984-10-17 | Hitachi Ltd | A semiconductor memory device and a method of manufacture thereof |
JPS6038855A (en) * | 1983-08-12 | 1985-02-28 | Hitachi Ltd | Semiconductor device and manufacture thereof |
EP0150597A1 (en) * | 1984-01-13 | 1985-08-07 | Kabushiki Kaisha Toshiba | Method of manufacturing semiconductor memory device having trench memory capacitor |
Family Cites Families (5)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JPS58137245A (en) * | 1982-02-10 | 1983-08-15 | Hitachi Ltd | Semiconductor memory and its manufacture |
JPS5982761A (en) * | 1982-11-04 | 1984-05-12 | Hitachi Ltd | Semiconductor memory |
JPS59106146A (en) * | 1982-12-10 | 1984-06-19 | Hitachi Ltd | Semiconductor memory |
JPS59191374A (en) * | 1983-04-15 | 1984-10-30 | Hitachi Ltd | Semiconductor integrated circuit device |
JPS6023506B2 (en) * | 1983-11-21 | 1985-06-07 | 株式会社日立製作所 | semiconductor storage device |
-
1985
- 1985-04-03 JP JP60070278A patent/JP2604705B2/en not_active Expired - Lifetime
-
1986
- 1986-04-02 EP EP86302450A patent/EP0197762B1/en not_active Expired
- 1986-04-02 DE DE8686302450T patent/DE3679698D1/en not_active Expired - Lifetime
-
1988
- 1988-03-21 US US07/171,177 patent/US4797719A/en not_active Expired - Lifetime
Patent Citations (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0118878A2 (en) * | 1983-03-07 | 1984-09-19 | Hitachi, Ltd. | Semiconductor memory device |
GB2138207A (en) * | 1983-04-15 | 1984-10-17 | Hitachi Ltd | A semiconductor memory device and a method of manufacture thereof |
JPS6038855A (en) * | 1983-08-12 | 1985-02-28 | Hitachi Ltd | Semiconductor device and manufacture thereof |
EP0150597A1 (en) * | 1984-01-13 | 1985-08-07 | Kabushiki Kaisha Toshiba | Method of manufacturing semiconductor memory device having trench memory capacitor |
Non-Patent Citations (1)
Title |
---|
PATENT ABSTRACTS OF JAPAN, vol. 9, no. 159 (E-326)[1882], 4th July 1985; & JP-A-60 038 855 (HITACHI) 28-02-1985 (Cat. X) * |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0317152A2 (en) * | 1987-11-13 | 1989-05-24 | Fujitsu Limited | Trench capacitor and method for producing the same |
EP0317152A3 (en) * | 1987-11-13 | 1990-09-12 | Fujitsu Limited | Trench capacitor and method for producing the same |
Also Published As
Publication number | Publication date |
---|---|
JPS61229349A (en) | 1986-10-13 |
JP2604705B2 (en) | 1997-04-30 |
EP0197762A3 (en) | 1987-08-19 |
US4797719A (en) | 1989-01-10 |
EP0197762B1 (en) | 1991-06-12 |
DE3679698D1 (en) | 1991-07-18 |
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